1. Field of the Invention
This invention relates generally to cameras adapted to use self developing film units that are successively exposed and then processed by being transported through a pressure applying assembly, and specifically to an improved pressure applying assembly for such a camera.
2. Description of the Prior Art
In a self-processing camera of the type to which the present invention relates, a multi-layered self developing film unit incorporating a multi-layered image receiving portion and a rupturable pod containing developing fluid is exposed in the camera and then advanced through a pressure applying assembly for developing. Pressure exerted by pressure applying members ruptures the pod containing the developing fluid and causes the fluid to flow between the layers of the image receiving portion to develop the image.
Pressure applying assemblies typically include a pair of elongate pressure applying members such as rollers held in a frame so that a certain amount of radial movement between the rollers is possible. The rollers are urged toward each other by means of a suitable spring. Once the exposed film unit is advanced into the nip formed by the rollers, continued movement of the film unit between the rollers spreads the fluid between the layers of the film unit. To obtain optimum development of the image, it is very critical that the developing fluid be spread evenly between the layers of the film unit throughout the image area. An important consideration in achieving even spreading of the developing fluid is that the pressure exerted by the pressure applying members be equally distributed across the width of the film unit as it is transported through the assembly.
Another important consideration in the design of a pressure applying assembly is its manufacturability. Ideally, the assembly must be easy to assemble, contain as few parts as possible, and require a minimum number of fine tolerances.
A good deal of the effort in designing pressure applying assemblies has gone into design of the spring or springs which yieldably urge the pressure applying members together. One approach has been to use a pair of U-shaped springs, one spring on either end of the pressure applying members, the members being retained between the arms of the U. Although this design has the advantage of ease of assembly, i.e., the forces of the springs are used to hold the members in the frame, the manufacturing tolerances of the forces exerted by the springs are quite critical since the springs must exert nearly equal force on the ends of the pressure applying members for even spreading of the developing fluid between layers in the film unit. It can be seen that if one of the springs were to exert more pressure on one end of the members than on the other, the distribution of developing fluid on that side of the film unit would be much thinner than on the other side resulting in uneven development of the image.
Another arrangement known in the art which automatically overcomes the disadvantages of unequal forces on the end of the pressure applying members is the use of a bow shaped spring, the ends of the spring being urged against the ends of one of the members by a force exerted on the center of the bow. Typically, the bow shaped spring is held against the pressure applying members by means of a part of the camera body or a part of the frame bearing on the central portion of the spring. The spring is held in position so that it can experience some rocking motion to thereby equalize the forces exerted by both ends of the spring. Although this arrangement has the advantage of automatically equalizing the spring forces on the ends of the pressure applying members, it suffers from the fact that it is not as easily assembled as the aforementioned configuration utilizing the U-shaped springs. Additionally, a very rigid frame or camera body must be provided to sustain the force exerted by the center of the bow-shaped spring against the frame or body of the camera.